This invention relates to a phosphorescent material for electroluminescent display. More particularly, it relates to improvements in the composition of the phosphorescent layer of a zinc sulfide powder phosphor electroluminescent display panel, such as a matrix display panel or a segmented display panel, especially such a panel intended for operation in the direct current (DC) mode; but also applicable to display panels intended for operation in the alternating current (AC) mode.
Electroluminescence is the emission of light from a crystalline phosphor due to the application of an electric field. A commonly used phosphor material is zinc sulfide activated by the introduction of less than one mole percent of various elements such as manganese into its lattice structure. When such a material is subjected to the influence of an electric field of a sufficient magnitude, it emits light of a color which is characteristic of the composition of the phosphor. Zinc sulfide activated with manganese (referred to as a zinc sulfide:manganese or ZnS:Mn phosphor) produces a pleasant yellowish orange centered at 585 nanometers (nm) wavelength.
ZnS:Mn phosphors are characterized by high luminance, luminous efficiency and discrimination ratio, and long useful life. Luminance is brightness or luminous intensity when activated by an electric field, and is commonly measured in lamberts, i.e. candelas per pi square centimeters, or in foot-lamberts, i.e. candelas per pi square feet. Luminous efficiency is light produced compared to power consumed by the device, commonly measured in lumens per watt. Discrimination ratio is the ratio of luminance in response to an "on" voltage to luminance in response to an "off" voltage.
A wide range of colors can be obtained by substituting or supplementing the manganese with other materials such as copper or alkaline earth activators, or by substituting or supplementing the zinc sulfide with other similar phosphorescent materials such as zinc selenide.
Phosphor materials can be formulated into a wide variety of electroluminescent configurations to serve numerous functions. In many electroluminescent devices the electroluminescent display is a panel which is divided into a matrix of individually activated pixels (picture elements).
Two major subdivisions of electroluminescent devices are AC and DC intended operating mode. In DC configurations, electrons from an external circuit pass through the pixels in the panel. In AC configurations, the pixels are capacitatively coupled to an external circuit.
Electroluminescent devices are also made using either powder or thin-film phosphor configurations. Powder phosphors are formed by precipitating powder phosphor crystals of the proper grain size, suspending the powder in a lacquer-like vehicle, and then applying the suspension to a substrate, for example by spraying, screening or doctor-blading techniques. Thin-film phosphors are grown from condensation of evaporants from vacuum vapor depositions, sputtering, or chemical vapor depositions.
Two configurations to which the present invention has high applicability are the powder phosphor electroluminescent matrix and segmented display panels, intended for operation in the direct current (DC) mode. Matrix display panels can be used for a variety of applications, and in general, can find utility as substitutes for cathode ray tubes (CRTs), wherever CRTs are used. For example, matrix display panels can be used for such applications as oscilloscopes, television sets and monitors for computers. A particularly advantageous application for the matrix display panel is as the monitor for a microcomputer, or personal computer. By avoiding the need for a CRT, an electroluminescent matrix display panel can make a personal computer more compact and thus more easily portable.
Segmented display panels find utility for example as alphanumeric displays in such apparatus as digital clocks; pocket calculators; and gasoline pump indicators for price, volume delivered and cost of amount delivered.
The use of electroluminescent matrix display panels as monitors for personal computers, and for various other applications, is known. Electroluminescent display panels, however, are subject to various modes of degradation after a period of use, and in due course the panels must be replaced.
It is, therefore, a purpose of this invention to provide an electroluminescent material for use in electroluminescent display panels, having increased useful life.